Beatrice,
this is kind of confusing. You say, you want to
"[...] keep[ing] one observation if there are duplicates for all make,
price and mpg."
You then go on, specifying rules for cases in which
"there are 2 duplicated positive price values when there is one
opposite negative price value for the common make and mpg"
But this is impossible. Given the first step, which elimintates all
but one positive (or negative) price value in the subgroup defined by
make and mpg, there can no longer be any cases that have 2 (or more)
duplicated positive (or negative) price values in terms of make and
mpg.
